1. Field of the Invention
The present invention relates to a technique of protecting a device for transmitting a signal using a signal line from an overvoltage of the signal line.
2. Description of the Related Art
As a technique of protecting a device for transmitting a signal using a signal line from an overvoltage of the signal line, a technique of connecting a signal line CL to a signal-transmission terminal 710 of a device 700 via a resistor 701, providing a diode 702 of which the cathode is connected to the terminal 710 of the device 700 and the anode is connected to the ground and a diode 703 of which the anode is connected to the terminal 710 of the device 700 and the cathode is connected to a power source, suppressing a current flowing in the terminal 710 using the resistor 701, suppressing a decrease of a voltage applied to the terminal 710 to a potential lower than the ground potential using the diode 702, and suppressing an increase of the voltage applied to the terminal 710 to a potential higher than the potential of the power source using the diode 703 is known as illustrated in FIG. 7A (for example, refer to JP 3-85016 A).
As a technique of protecting a device 700 for transmitting a signal using a signal line from an overvoltage of the signal line, a technique of providing a Zener diode 712 of which the cathode is connected to a signal-transmission terminal 710 of the device 700 and the anode is connected to the ground, suppressing a current flowing in the terminal 710 using a resistor 711, and suppressing an increase of a voltage applied to the terminal 710 to a potential higher than a Zener voltage of the Zener diode 712 using a Zener diode 712 is known as illustrated in FIG. 7B (for example, JP 2009-71373 A).
According to the technique illustrated in FIG. 7A, when an overvoltage is generated in the signal line CL and a current flowing into the power source via the diode 703 is larger than current consumption in the device 700, the potential of the power source of the device 700 may be higher than a rated voltage of the device 700 and thus the device 700 may be destroyed.
According to the technique illustrated in FIG. 7B, when the power source of the device 700 is turned off and an overvoltage is generated in the signal line CL, the voltage applied to the terminal 710 may be higher than the absolute rated voltage and thus the device 700 may be destroyed.
That is, for example, when it is assumed that the absolute rated voltage of the terminal 710 is in a range between the ground potential −0.5 V and the power source potential +0.5 V, the power source voltage when the power source is turned on is 5 V, and the Zener voltage of the Zener diode 712 is 5 V, the absolute rated voltage in a state in which the power source is turned on is in a range between −0.5 V and 5.5 V (5 V+0.5 V). When the power source is turned on, the voltage applied to the terminal 710 is limited to the Zener voltage 5 V or less and thus the voltage applied to the terminal 710 is in the range of the absolute rated voltage. On the other hand, when the power source of the device 700 is turned off and the potential of the power source is 0 V, the absolute rated voltage is in a range between −0.5 V and +0.5 V. Accordingly, even when the voltage applied to the terminal 710 is limited to the Zener voltage 5 V or less, the voltage applied to the terminal 710 cannot be limited to the range of the absolute rated voltage.
In addition, there is a problem in that a great individual difference is present in the Zener voltage of a general Zener diode.